The present invention is directed to catalyst compositions and processes for reacting toluene and methanol to form styrene.
Styrene is currently commercially produced from benzene in a two-step process. In the first step benzene is alkylated with ethylene to form ethylbenzene, and in the second step, the ethylbenzene is dehydrogenated to form styrene.
For example, the alkylation of aromatic compounds with olefins, alkyl halides and alcohols in the presence of a rare earth metal (including cerium) modified X- or Y-type zeolite is broadly disclosed in U.S. Pat. No. 3,251,897. Such alkylations are non-specific to styrene, the predominant reaction disclosed being benzene + ethylene to form ethylbenzene. Thus, such zeolite catalyzed reactions can be employed to make ethylbenzene in the first stage of conventional styrene synthesis.
One of the known alternative routes for forming styrene involves the oxidative coupling of toluene to form 1,2-diphenyl ethylene (stilbene) followed by the disproportionation of the stilbene with ethylene in the presence of a catalyst to form styrene. The economic significance of the overall process scheme of the toluene-stilbene route resides in the fact that styrene can be produced from 0.5 mole of ethylene and one mole of toluene. This compares with the conventional ethylbenzene route wherein styrene is produced from one mole of ethylene and one mole of benzene.
In light of the rising costs of benzene and ethylene and the environmental problems of benzene, toluene-based processes will become a more attractive route than the existing benzene-based process for styrene manufacture.
Representative catalysts employed in the toluene to stilbene route are metal oxides such as those disclosed in U.S. Pat. Nos. 3,694,518; 3,739,038; 3,868,427; 3,965,206; 3,980,580; 4,091,044; 4,183,828; 4,243,825; 4,247,727; 4,254,293; 4,255,602; 4,255,603; 4,255,604; 4,268,703; 4,268,704; 4,278,824; 4,278,825; and 4,278,826 all assigned to Monsanto.
Commonly assigned U.S. Pat. application Ser. No. 405,803, filed Aug. 6, 1982 by H. Teng and I. Huang employs a faujasite zeolite modified with Li, K, Rb or cesium cations and at least one promoter selected from the group consisting of B, P, Pb, Cu, Zn, Ni, O, and Fe for the toluene to stilbene route.
A separate and distinct alternative route to styrene from toluene involves the alkylation of the side chain of toluene with methanol or formaldehyde by contact of these reactants with X- or Y-type zeolites, as described in Yashima et al in the Journal of Catalysis, Vol. 26, 303-312 (1972). More specifically, it is disclosed therein that alkylation of the methyl group of toluene to form styrene and ethylbenzene is effected by Na, K, Rb or Cs exchanged X- or Y-type zeolites, whereas Li exchanged zeolites of the same type effected predominantly alkylation of the benzene ring of toluene to form xylenes. Yashima et al interpret their results as suggesting that xylene formation is attributable to the acidity of the catalyst, whereas styrene and ethylbenzene formation is attributable to the basicity of the catalyst.
Sidorenko et al in the article "Condensation of Toluene and Methanol on Synthetic Zeolites Exchanged with Alkali Ions", Dokl. Akad. Nauk SSSR, Vol. 173 No. 1:132-34 (1967), have proposed a mechanism for the alkylation of toluene with methanol using alkali metal exchanged X- and Y-type zeolites wherein methanol is converted to formaldehyde which then reacts with toluene to produce styrene and ethylbenzene.
However, since alkali metal exchanged zeolites are capable of catalyzing a variety of reactions and therefore produce a variety of by-products, the selectivity of the toluene to styrene is very low when conducting the process in accordance with Yashima et al or Sidorenko et al.
Furthermore, the commercial attractiveness of the toluene/methanol alkylation reaction is contingent upon achieving a sufficiently high selectivity to styrene relative to ethylbenzene to reduce the cost of dehydrogenating ethylenebenzene to styrene.
In an effort to improve the selectivity of the toluene/methanol alkylation reaction to styrene, Unland et al, U.S. Pat. No. 4,140,726 describe the use of an X- or Y-type zeolite which has been modified by a cation exchange with one or more of potassium, rubidium and cesium and impregnated with boron or phosphorus. While toluene to methanol mole ratios in the feed are disclosed as varying from 0.5:1 to 20:1 (Col. 2, Lines 20 et seq) it is disclosed to be desirable to employ excess toluene in the feed relative to methanol to minimize side reactions which decompose methanol, and in fact the data reported in the examples employ toluene:methanol ratios of at least 5:1. Conversions and selectivities are reported on the basis of methanol in the feed. A disadvantage is associated with maintaining the methanol concentration in the feed substantially below the toluene concentration, namely, the effective toluene conversion per pass at a toluene:methanol mole ratio of 5:1 is necessarily not greater than 20% of the methanol conversion for stoichiometric reasons and in this patent can be calculated to be 6.7% (Example 1, Col. 6, Line 5). Toluene is more expensive than methanol, and ideally one would want to maximize toluene conversion and avoid or minimize recycling of the toluene. To do this, however, the methanol concentration in the feed must be increased. Recycle of methanol relative to toluene is a much simpler procedure and due to the cost of methanol such recycle may even be dispensed with altogether. If one attempts to improve toluene conversion by increasing the methanol concentration in the feed when employing the Unland et al catalysts, however, the styrene selectivity ratio, i.e. the ratio of styrene to styrene + ethylbenzene on a mole percentage basis, is reduced as described hereinafter in the Comparative Example. Thus, when using the Unland et al catalysts one is forced to accept a substantial reduction in the styrene selectivity ratio for increases in toluene conversions induced by increases in the methanol concentration.
Itoh et al report in J. of Catalysis, Vol. 72, p. 170 (1981) the use of Rb, K, Li cation exchanged X-type zeolites, such as Rb Li-X, Rb-X and RbK-X, for the side chain alkylation of p-xylene with methanol to produce p-methylstyrene and p-ethyltoluene. A maximum 68 mole % conversion of methanol with mole % yields of 5.3% (p-methyl styrene) and 2.7% (p-ethyltoluene) are disclosed. Use of Cs and/or Li exchange in conjunction with B and/or P impregnated zeolites is not disclosed nor the effects obtainable therefrom.
Japanese Patent Application Publication No. Sho 57-68144 published Apr. 26, 1982 is directed to catalyst for styrene synthesis which comprises a zeolite of the faujasite class having at least 20% of the sodium cations present therein exchanged with cesium, potassium or rubidium and which has been treated to impregnate therein one or more divalent or trivalent metal salts of boric or phosphoric acid, the metal of said salt disclosed as being selected from magnesium, calcium, aluminum, magnanese, iron, cobalt, nickel, copper and zinc.
The search has continued for catalyst compositions capable of improving the conversion and/or styrene selectivity of toluene side chain alkylation reactions with methanol. The present invention was developed in response to this search.